What is that union which the elect have with Christ?
The union which the elect have with Christ is the work of God's grace,1 whereby they are spiritually and mystically, yet really and inseparably, joined to Christ as their head and husband;2 which is done in their effectual calling.3
Scripture proofs
The Westminster Assembly's proof texts (KJV). The bracketed numbers in the answer above mark the clause each set of references supports; expand a row to read the verses.
1 Eph. 1:22; Eph. 2:6–8
And hath put all things under his feet, and gave him to be the head over all things to the church,
Eph. 2:6 And hath raised us up together, and made us sit together in heavenly places in Christ Jesus:
Eph. 2:7 That in the ages to come he might shew the exceeding riches of his grace in his kindness toward us through Christ Jesus.
Eph. 2:8 For by grace are ye saved through faith; and that not of yourselves: it is the gift of God:
2 1 Cor. 6:17; John 10:28; Eph. 5:23, 30
But he that is joined unto the Lord is one spirit.
And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand.
Eph. 5:23 For the husband is the head of the wife, even as Christ is the head of the church: and he is the saviour of the body.
Eph. 5:30 For we are members of his body, of his flesh, and of his bones.
3 1 Pet. 5:10; 1 Cor. 1:9
But the God of all grace, who hath called us unto his eternal glory by Christ Jesus, after that ye have suffered a while, make you perfect, stablish, strengthen, settle you.
God is faithful, by whom ye were called unto the fellowship of his Son Jesus Christ our Lord.
This doctrine across the Standards
Where the Confession and the Westminster Shorter Catechism treat the same matter, so the question can be studied alongside its parallel statements.
In the Westminster Confession
VIII.8 To all those for whom Christ hath purchased redemption he doth certainly and effectually apply and communicate the …
To all those for whom Christ hath purchased redemption he doth certainly and effectually apply and communicate the same; making intercession for them, and revealing unto them, in and by the Word, the mysteries of salvation; effectually persuading them by his Spirit to believe and obey; and governing their hearts by his Word and Spirit; overcoming all their enemies by his almighty power and wisdom, in such manner and ways as are most consonant to his wonderful and unsearchable dispensation.
→ VIII.8 · Of Christ the MediatorIX.4 When God converts a sinner, and translates him into the state of grace, he freeth him from his …
When God converts a sinner, and translates him into the state of grace, he freeth him from his natural bondage under sin, and by his grace alone enables him freely to will and to do that which is spiritually good; yet so as that, by reason of his remaining corruption, he doth not perfectly, nor only, will that which is good, but doth also will that which is evil.
→ IX.4 · Of Free WillX.1 All those whom God hath predestinated unto life, and those only, he is pleased, in his appointed and …
All those whom God hath predestinated unto life, and those only, he is pleased, in his appointed and accepted time, effectually to call, by his Word and Spirit, out of that state of sin and death, in which they are by nature, to grace and salvation by Jesus Christ; enlightening their minds, spiritually and savingly, to understand the things of God; taking away their heart of stone, and giving unto them an heart of flesh; renewing their wills, and by his almighty power determining them to that which is good, and effectually drawing them to Jesus Christ; yet so as they come most freely, being made willing by his grace.
→ X.1 · Of Effectual CallingX.2 This effectual call is of God's free and special grace alone, not from anything at all foreseen in …
This effectual call is of God's free and special grace alone, not from anything at all foreseen in man; who is altogether passive therein, until, being quickened and renewed by the Holy Spirit, he is thereby enabled to answer this call, and to embrace the grace offered and conveyed in it.
→ X.2 · Of Effectual CallingX.3 Elect infants, dying in infancy, are regenerated and saved by Christ through the Spirit, who worketh when, and …
Elect infants, dying in infancy, are regenerated and saved by Christ through the Spirit, who worketh when, and where, and how he pleaseth. So also are all other elect persons who are incapable of being outwardly called by the ministry of the Word.
→ X.3 · Of Effectual CallingX.4 Others, not elected, although they may be called by the ministry of the Word, and may have some …
Others, not elected, although they may be called by the ministry of the Word, and may have some common operations of the Spirit, yet they never truly come unto Christ, and therefore cannot be saved: much less can men, not professing the Christian religion, be saved in any other way whatsoever, be they never so diligent to frame their lives according to the light of nature and the law of that religion they do profess; and to assert and maintain that they may is very pernicious, and to be detested.
→ X.4 · Of Effectual CallingIn the Westminster Shorter Catechism
Q.29 How are we made partakers of the redemption purchased by Christ?
We are made partakers of the redemption purchased by Christ, by the effectual application of it to us by his Holy Spirit.
→ Open Q.29Q.30 How doth the Spirit apply to us the redemption purchased by Christ?
The Spirit applieth to us the redemption purchased by Christ, by working faith in us, and thereby uniting us to Christ in our effectual calling.
→ Open Q.30Q.31 What is effectual calling?
Effectual calling is the work of God's Spirit, whereby, convincing us of our sin and misery, enlightening our minds in the knowledge of Christ, and renewing our wills, he doth persuade and enable us to embrace Jesus Christ, freely offered to us in the gospel.
→ Open Q.31Q.32 What benefits do they that are effectually called partake of in this life?
They that are effectually called do in this life partake of justification, adoption, and sanctification, and the several benefits which in this life do either accompany or flow from them.
→ Open Q.32